In a conventional thermal printing apparatus, a thermal head presses a surface of a thermo-transfer ribbon coated with an ink to a print paper and heats the ink until it is melted. The melted ink is torn from the ribbon and is transferred to the print paper, so that a desired character is printed on the paper.
As such conventional thermal printing apparatus, a color printing apparatus is known, wherein the ink of each color among three primary colors, namely yellow, magenta and cyan, is sequentially transferred to the print paper in a superposed fashion, and thereby a multi-color printing operation is performed. The operation for multiple-color printing includes a first process of transferring the ink of one primary color to the print paper and a second process of transferring the ink of another primary color to the paper having previously had ink transferred thereto, in a superposed fashion. A printing speed at which the thermal head is moved relative to the print paper, is set to be equal in both processes.
Upon transferring the ink, however, adhesive force between the print paper and the ink is different from adhesive force between the ink transferred to the paper and the ink to be superposed. For instance, in the case where an ink of the thermo-transfer ribbon is constituted with a coloring agent and a binder of waxy substance such as paraffin wax, carnauba wax and castor wax, the ink melted by the thermal head has relatively strong adhesive force to the print paper, because the melted ink partly soaks into the paper. But, the adhesive force of the melted ink to the transferred ink is not strong, because the agglomerating force of the melted ink with the transferred ink is weak owing to low viscosity of the melted ink. On the other hand, in the case where an ink of the thermo-transfer ribbon is constituted with a coloring agent and a binder of resinous substance such as rosin and ethylene-vinyl acetate copolymer resin, the adhesive force of the melted ink to the transferred ink is strong, because the agglomerating force of the melted ink with the transferred ink is strong owing to high viscosity of the melted ink. But, the adhesive force of the ink to the print paper is not strong, because little melted ink soaks into the paper.
Thus, the adhesive force of the ink is different depending on a material of the print medium and a property of the ink. If the ink having the weak adhesive force to the print medium is used and the printing operation is performed at a normal speed, insufficient ink is transferred to the print medium, so that the required printing cannot be performed. Namely, if the first process and second process are performed at the same printing speed and the ink constituted with waxy substance is used, the agglomerating force of the ink melted in the second process is weak when the ink is not chilled sufficiently, so that insufficient ink is torn from the thermo-transfer ribbon. Consequently, there occurs defective transference such as printing with dimmed characters.